> ## Documentation Index
> Fetch the complete documentation index at: https://upstash.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# EVAL_RO

> Execute read-only Lua script.

Use `EVAL_RO` to run a Lua script that is not allowed to write.

It behaves like [`EVAL`](/docs/redis/commands/scripting/eval), with `<numkeys>` splitting the arguments into the `KEYS` and `ARGV` tables, except that any write command called from the script fails with an error. Declaring the read-only intent lets the server serve the call on replicas and turns an accidental write into a clear error instead of an unexpected modification, which is worth doing for every script that only computes over existing data.

Being read-only is not by itself enough to run concurrently with other commands: like `EVAL`, this command takes the global lock unless the script's shebang sets the `allow-key-locking` flag. With the flag, the script takes shared read locks on the keys passed in `KEYS`, so several readers of the same key proceed together. Combine the two flags with a comma, as in `#!lua flags=no-writes,allow-key-locking`, and see [Key-Based Locking](/docs/redis/features/key-locking) for the full rules.

## Syntax

```redis theme={"system"}
EVAL_RO <script> <numkeys> [<key> [<key> ...]] [<arg> [<arg> ...]]
```

## Arguments

| Argument    | Required | Repeatable | Description                                             |
| ----------- | -------- | ---------- | ------------------------------------------------------- |
| `<script>`  | Yes      | No         | Lua script source.                                      |
| `<numkeys>` | Yes      | No         | Number of key arguments that follow.                    |
| `<key>`     | No       | Yes        | Redis key targeted by the command.                      |
| `<arg>`     | No       | Yes        | Additional argument, available to the script as `ARGV`. |

## Important points

* `numkeys` must equal the number of key arguments that immediately follow it; remaining arguments are available to the script or function as ordinary arguments.
* A read-only script still takes the global lock unless its shebang sets the `allow-key-locking` flag. See [Key-Based Locking](/docs/redis/features/key-locking).
* Pass every key the script reads through `KEYS` whether or not `allow-key-locking` is set. A key built inside the script is read from disk under the lock when it is not in memory, and it is rejected outright when the flag is set. See [Dynamic Keys and Latency](/docs/redis/features/key-locking#dynamic-keys-and-latency).

## Response

The reply reports the result of the operation. Error replies have the same shape in RESP2 and RESP3 and are surfaced as exceptions by the SDKs below.

| Protocol | Reply                                            |
| -------- | ------------------------------------------------ |
| RESP2    | Reply produced by the evaluated read-only script |
| RESP3    | Reply produced by the evaluated read-only script |

<Note>
  Client libraries often decode bulk strings, maps, sets, and numeric strings into language-native values. The table describes the Redis wire reply.
</Note>

## Examples

TCP examples use the TLS `REDIS_URL` from the Upstash console. REST examples use `UPSTASH_REDIS_REST_URL` and `UPSTASH_REDIS_REST_TOKEN`.

<AccordionGroup>
  <Accordion title="Redis CLI" icon="terminal">
    ```bash theme={"system"}
    EVAL_RO "return ARGV[1]" 0 hello
    ```
  </Accordion>

  <Accordion title="@upstash/redis" icon="node-js" iconType="brands">
    ```ts theme={"system"}
    import { Redis } from "@upstash/redis";

    const redis = Redis.fromEnv();

    const script = `
        return ARGV[1]
    `
    const result = await redis.evalRo(script, [], ["hello"]);
    console.log(result) // "hello"
    ```
  </Accordion>

  <Accordion title="upstash_redis" icon="python" iconType="brands">
    ```python theme={"system"}
    from upstash_redis import Redis

    redis = Redis.from_env()
    result = redis.eval_ro("return ARGV[1]", args=["value"])
    print(result)
    ```
  </Accordion>

  <Accordion title="ioredis" icon="node-js" iconType="brands">
    ```ts theme={"system"}
    import Redis from "ioredis";

    const redis = new Redis(process.env.REDIS_URL!);
    const result = await redis.eval_ro("return ARGV[1]", "0", "value");
    console.log(result);
    ```
  </Accordion>

  <Accordion title="node-redis" icon="node-js" iconType="brands">
    ```ts theme={"system"}
    import { createClient } from "redis";

    const client = await createClient({ url: process.env.REDIS_URL })
      .on("error", console.error)
      .connect();
    const result = await client.evalRo("return ARGV[1]", { arguments: ["value"] });
    console.log(result);
    ```
  </Accordion>

  <Accordion title="redis-py" icon="python" iconType="brands">
    ```python theme={"system"}
    import os
    import redis

    client = redis.from_url(os.environ["REDIS_URL"])
    result = client.eval_ro("return ARGV[1]", 0, "value")
    print(result)
    ```
  </Accordion>

  <Accordion title="go-redis" icon="golang" iconType="brands">
    ```go theme={"system"}
    package main

    import (
        "context"
        "fmt"
        "os"

        "github.com/redis/go-redis/v9"
    )

    func main() {
        opts, err := redis.ParseURL(os.Getenv("REDIS_URL"))
        if err != nil {
            panic(err)
        }
        client := redis.NewClient(opts)
        result, err := client.EvalRO(context.Background(), "return ARGV[1]", nil, "value").Result()
        if err != nil {
            panic(err)
        }
        fmt.Println(result)
    }
    ```
  </Accordion>

  <Accordion title="jedis" icon="java" iconType="brands">
    ```java theme={"system"}
    import java.net.URI;

    import redis.clients.jedis.Jedis;

    try (Jedis jedis = new Jedis(new URI(System.getenv("REDIS_URL")))) {
      Object result = jedis.evalReadonly("return ARGV[1]", java.util.List.of(), java.util.List.of("value"));
      System.out.println(result);
    }
    ```
  </Accordion>

  <Accordion title="redis-rs" icon="rust" iconType="brands">
    ```rust theme={"system"}
    fn main() -> redis::RedisResult<()> {
        let url = std::env::var("REDIS_URL").expect("REDIS_URL is not set");
        let client = redis::Client::open(url)?;
        let mut connection = client.get_connection()?;

        let mut command = redis::cmd("EVAL_RO");
        command.arg("return ARGV[1]");
        command.arg("1");
        let result: redis::Value = command.query(&mut connection)?;
        println!("{result:?}");
        Ok(())
    }
    ```
  </Accordion>
</AccordionGroup>


## Related topics

- [EVAL_RO](/docs/redis/sdks/py/commands/scripts/eval_ro.md)
- [EVALSHA_RO](/docs/redis/sdks/py/commands/scripts/evalsha_ro.md)